Slurry phase polymerisation of olefins is well known wherein an olefin monomer, and optionally olefin comonomer, are polymerised in the presence of a catalyst in a diluent in which the solid polymer product is suspended and transported.
This invention is particularly related to polymerisation in a loop reactor where the slurry is circulated in the reactor typically by means of a pump or agitator. Liquid full loop reactors are well known in the art and are described for example in U.S. Pat. Nos. 3,152,872, 3,242,150 and 4,613,484. Polymerisation is typically carried out at temperatures in the range 50-125° C. and at pressures in the range 1-100 bara. The catalyst used can be any catalyst typically used for olefin polymerisation such as chromium oxide, Ziegler-Natta, metallocene or late transition metal-type catalysts. In slurry phase olefin polymerisation processes, the product is withdrawn from the polymerisation reactor in the form of a slurry of solid olefin polymer product suspended in a diluent. The diluent may be inert or reactive, liquid or supercritical. The product slurry comprising polymer and diluent, and in most cases catalyst, olefin monomer and comonomer, can be discharged intermittently or continuously.
In many polymerisation processes, concentrating devices such as hydrocyclones or settling legs are used to minimise the quantity of fluids withdrawn with the polymer for downstream reaction or processing. Such devices separate the polymer-containing stream into a solids-rich stream and a solids-lean stream. However whilst centrifugal concentrating devices are well known, the design of such devices itself is typically such that the operation can only be optimised over a limited range of design operating conditions. This is particularly the case for fixed concentrating devices which contain no rotating parts (the speed of which can be varied based on throughput). When operating away from such design operating conditions, particularly during start-up or grade transitions, not only is the efficiency of the solids concentration and particle size separation compromised but also the concentrating device and its associated pipework become much more susceptible to blockage, mainly due to polymerisation of active polymer present in the stream being concentrated. It is known in U.S. Pat. No. 3,816,383 to recycle part of the solids-rich stream produced by a concentrating device directly back into the polymerisation zone; however this process does not provide a means of varying the solids concentration of the feed to the concentrator independently of the solids concentration in the off-take stream from the polymerisation zone. In particular it cannot provide the concentrator with a feed stream that is more concentrated in solids than in the stream withdrawn from the polymerisation zone.
EP 1118626A discloses a process in which the solids-lean stream produced by a concentrating device is recycled directly back into the polymerisation zone. As above, this does not permit any control of the concentration of the feed entering the concentrator itself.